I don't have Windows 8, but I had to set up my partner, who lives out of state, with a new laptop. I bought him a Windows 8 one (non-touchscreen) and spent a few weeks setting it up for him.
His needs are very simple. All he does on his computer is go to websites and check e-mail, and open documents once in a while. Because of this, it seemed to me that the Metro mode would be a good bet for him.
I love my Windows Phone, and esthetically I like the whole Metro look very much. Unfortunately, the Metro apps for IE and Mail are so stripped down that they're basically useless.
On IE, your Favorites show up as one long ribbon, and you can't categorize them in folders. So if you have 30 or 40 Favorites (as most normal people do, at least), you have to scroll, scroll, scroll through them to find the one you want. Instead of doing that, I pinned all his Favorites to the Start screen as tiles, since on the Start screen you can group tiles by category. (That took a couple of hours.) But that's not a great solution because when he wants to go to a site he has to constantly jump out of IE back to the Start screen to access his Favorites tiles.
On Mail, you cannot click on a sender in an e-mail and choose to add that person to your People list (contacts). This is a basic function which just about everyone needs to do. So if you want to add someone, you have to write down their e-mail address, then go to the People app and manually type in the address.
In Metro apps in general, the design concept is to present a screen with as few on-screen controls as possible, so that you have to click (or gesture on a touchscreen) in order to access anything. So in IE you need to right-click on the screen in order to see open tabs. In Mail, there is a single "+" on the screen to start a new e-mail, but for everything else you have to right-click to bring up the options. The minimalist look is certainly esthetically pleasing, but it's hard to work in that environment - the extra mouse click or gesture to bring up options makes it feel like you have a veil between you and the computer that wasn't there before.
My partner gave it a shot, but after a week of using it he was unhappy. I had already told him that we could always use the desktop mode, so that's what we did, and it's much easier for him now.
The desktop mode isn't perfect either, though. You can't easily set a program to auto-start on bootup anymore (from searching the Internet I think there is a way to do it, but it looks so complicated, I'm afraid to try it). And yes, I do think there needs to be a Start button. Basic functions like accessing the Control Panel or shutting down the computer require you to memorize commands, rather than hitting "Start" and choosing what you want.
I really wanted to like the Metro mode but it just doesn't seem usable at this point.